Travelers troubled by rising airfares, canceled flights, and overcrowded tarmacs are hearing yet another reason to reconsider air travel.

Some say it's unethical to fly.

Earlier this month, neighborhood and environmental activists staged events across Britain to dramatize concerns about commercial aviation. Donning masks of Prime Minister Gordon Brown and waving cardboard airplanes, they called on government to keep track of carbon emissions from planes and raise fees to discourage frequent flying.

Behind this action lurks an ethics-based argument that's trying to shame routine fliers in developed nations into flying less. The nub: The planet should not have to suffer the consequences of a fast-growing (if now troubled) air-travel industry. Hence, the argument goes, an ethical consumer should think twice before buying plane tickets.

"If we're going to reduce aviation's contribution to climate change, then the onus is on people in the rich world to look at their flying habits," says John Stewart, chair of AirportWatch, a Britain-based coalition to curtail flying and airport expansion. That's because most fliers don't live in developing nations, he says.

Estimates for significant growth in air travel are fueling today's ethics debates. The World Tourism Organization projects the number of international leisure travelers to nearly double from 842 million in 2006 to 1.6 billion in 2020. Most of those travelers are expected to go by air.

Science hasn't put the ethical issue to rest. Airplane emissions currently account for about 3 percent of greenhouse-gas emissions worldwide, according to Daniel Sperling, director of the Institute of Transportation Studies at the University of California, Davis. He says taking a train across the United States generates about 20 percent fewer emissions than an average cross-country flight. But doing the trip solo in a car would produce about 66 percent more carbon per passenger mile than an average flight.

That flying has a detrimental effect on the environment is widely accepted. The ethical debate hinges instead on such questions as: How much damage is acceptable? When is a flight justified? And when do the benefits of cross-cultural interaction, made possible by flying, outweigh the costs borne by the environment and those who live near runways?

Moral authorities of varied stripes have weighed in. In 2006, London's Anglican Bishop John Chartres said flying abroad to vacation is a "symptom of sin" because it ignores "an overriding imperative to walk more lightly upon the earth." Environmentalists have also framed flying as a moral issue since it allegedly causes harm in pursuit of unnecessary ends. "You can be an environmental saint – drive a hybrid car, recycle, conserve your water – and if you take one air flight, it actually blows your carbon budget right out of the water," says Elle Morrell, director of a green-lifestyle program at the Australian Conservation Foundation. One round-trip flight from Sydney to New York City, she says, generates as much in carbon-dioxide emissions per passenger as an average Australian would generate in an entire flightless year.

"We ask people to take this seriously," Ms. Morrell says, "and avoid air travel where they possibly can."

Against the prospect of vilification, the airline industry is pushing back. The Air Transport Association, a trade group whose members include most US carriers, contends the industry is constantly improving fuel efficiency and reducing noise. And employing some 11.4 million people may have some ethical value in its own right, says ATA spokesperson David Castelveter. "Would it be a logical or practical recommendation to suggest that people fly less, given the amount of jobs and economic activity that the aviation industry drives?" Mr. Castelveter says. "We say the answer is, 'No. Allow us to continue to focus on ways to reduce emissions.' "

"Of everything involved in tourism, airplane travel is doing the most damage in terms of climate change. That's absolutely true," Ms. Honey says. "But the movement in Europe saying, 'Stay home; don't get on a plane' is disastrous for poor countries … whose most important source of income is from nature-based tourism. It's also disastrous for us as a human race to not travel and see the world. The question is, 'How do you do it, and do it smartly?' "

Honey recommends taking other steps to minimize climate impacts. Once in a destination, she says, travelers may opt for energy-efficient ground transportation. They can also buy carbon offsets, which usually support either tree-planting initiatives or alternative-energy sources, in an attempt to neutralize the environmental impact of their journeys.

Some advocates for responsible travel, however, remind fliers that offsets don't neatly and easily remove the carbon generated by their jaunts.

"Offsetting is too often used as a bargaining tool [with one's conscience] to say 'Hey, I can fly, I just have to offset,' " says Tricia Barnett, director of Tourism Concern, a Britain-based advocacy organization for local peoples and environments affected by travel. "That's not necessarily a solution." She encourages fliers to also make extra efforts on their trips to eat locally raised foods, use public transportation, and limit water use.

At the Climate Institute, a Washington, D.C.-based group focused on climate-change solutions, Director John Topping feels no great need to make fliers feel guilty. He sees the marketplace as already driving some behaviors that ease pressure on climate change. Business travelers save money by hosting virtual meetings, he says, and short-distance fliers find they can sometimes spend less time and money on travel by riding buses and avoiding airports. Looking to the future, Virgin Atlantic airlines is exploring the use of biofuels in planes. For now, fliers are limited to those powered by petroleum-based jet fuels.

But since Americans generally drive cars more than they fly, some advocates suggest they fix their road habits first.